Various factors which influence the activity of the renin-angiotensin-aldosterone system are being studied as part of an investigation to determine the cause of low renin hypertension. These studies show that somatostatin and prolactin have little or no effect. Age, diabetes and drugs inhibiting the prostaglandin system may have profound effect and, in fact, the prostaglandin system appears to play a major role in regulating renin release. This role was demonstrated by both in vivo or in vitro techniques. Blockade of the prostaglandin production may lead to hyporeninemic-hypoaldosterone syndrome with secondary hyperkalemia.